Spinning Street Roadster

[V4F STR 60]

As a newbie V4F/STR racer with a sticker, I have to ask a newbie-with-a-sticker question.   At what speed do aerodynamics (CP/CG, lift, etc.) become a legitimate factor in a roadster?  I'm trying to discern whether my issues are aero, chassis/suspension, me... or all of the above.  There are no stupid questions, right?

Be kind, Pedro

Godspeed Cliff Gullett

[Stan Back]

We were going about 185 on our first run with a posi in our C/STR when we got legitimate factors and spun for the first time in our 7 years.   What legitimate factors -- don't know.  Took JD's advise --"if you spin with a posi, take it out.  If you spin with an open, put in a posi" or words to that effect.

Took it out -- next two runs were record runs at 210.

(By the way, you'd truckstop if you saw the diversity of wheel weight readings on this car.)

Stan Back

[Sumner]

As a newbie V4F/STR racer with a sticker, I have to ask a newbie-with-a-sticker question.   At what speed do aerodynamics (CP/CG, lift, etc.) become a legitimate factor in a roadster?  I'm trying to discern whether my issues are aero, chassis/suspension, me... or all of the above.  There are no stupid questions, right?

Be kind, Pedro

Godspeed Cliff Gullett

How about more specifics about the car and the condition?  Total weight, front/rear distribution, HP, spun at what speed, spun under acceleration or braking, etc..

Sum

[V4F STR 60]

OK, here goes-

1928 Dodge Roadster V4F/STR #60.  New car.  Total of 6 trips down the salt the year.

Weight 2393 # with driver.  53.1 % rear weight distribution.  No ballast.

Car was traveling 117 mph (according to the GPS) just before the 2 mile and it just, literally, turned around.  I could definitely feel the wind causing the car to wander, but it literally, just turned around.  It was not an overcorrection... as I didn't even have time to correct.  Again, it just turned around.   Conditions at time of spin.  Big wind.  About 29 mph.  I was the last run before they shut it down for the day because of the wind.

But, that being said, the car was a handful every run before.  The rear end seemed to want to hop from side to side.  After discussing this with Ron Joliffe in impound he suggested more rear tire (Goodyear Front Runner 27 X 4.5) pressure (was 45 psi).  I did increase it to 60 psi for my return run, and it did seem to help the hula dance a bit, but it did not eliminate it.  FYI- have a background in sports car racing, autocross and hill climbing so I have a pretty good understanding of vehicle dynamics, but I have never experienced such instantaneous, abrupt oversteer as I did on the last run of the week (the one where I got the sticker from Lee Kennedy!).  Kind of an embarrassing way to start your landspeed career.

Since returning, I have done the following-

Checked the rear suspension.  Ford 9" (Currie) Rear axle is square in the chassis.  No bind.  I have about 1 1/2 in travel in compression but very little in rebound (had to jack the coilovers up at the last second to add the fenders for STR.  Wondering if this zero rebound situation "lifted' the rear end.  I have ordered new coilovers to allow for more rebound.

Front end.  Solid front axle.  6" drop.  All new components.  Transverse spring  1/16 inch toe in.  9 degrees caster.  Chassis mounted rack, but negligible bump steer.  No front panhard.  1 1/2 compression, plenty of rebound.  No bind.  15 X 4" wheels 1 3/4 back space.  4" scrub radius.

I've read every posting I could find on suspension since (hindsight...) and based on what I read, this week I changed steering arms to greatly reduce Ackerman, added a dead perch to eliminate any shackle deflection, and added another 1/16 inch toe in (total 1/8).

Any suggestions are greatly appreciated!

Thanks, Pedro

[jimmy six]

As described by Roadster above, spins appear to become a big factor when the rear tires break loose. Our roadster did that before I added about 600#. No more breaking loose.

The next fun time is when you run out of HP or when the torque falls off and you have just past the 4 and have a mile to go. When hooked up and pulling things are great and you are a very happy driver. When the HP runs out and you are no longer accerating hard those flatnose roadsters tell their driver "let's see if you are a real driver and take you from the race track to the dance floor".
My first words in my head just after passing the 4 mile was "holy s--t". and don't spin it out. Ran out of HP. Gained 1.5 mph for the last mile and I can tell you that's not enough for a good ride anly a wide awake one.

The answer for us.....more gear.............Good Luck
 

[Sumner]

...............  4" scrub radius.............

That is the first thing I would get right and as JD has said a bad handling car can appear to not be so bad handling while accelerating, but become a lot more noticeable at a more constant speed.  It looks like from your speeds (on SCTA site) you were at pretty much the same speed for a lot of your run.

I don't know what your HP is, but I'll bet you have enough weight for the HP you are making.  If you can get more weight up front and it doesn't hurt your mph it sure won't hurt anything.

You didn't really mention that you were spinning the tires, so I don't think that is what upset the car.

I glad you posted since we can all learn from what is happening to others.

c ya,

Sum

[V4F STR 60]

I am intrigued by the "running out of HP" comments in this post and others.  The car was definitely "out of gear".  The cars was maxed out by the 2 mile every run, and even more so on the "spin run" due to the wind.  And now that you mention it, it also seemed to "hula dance" the most near max rpm.  We never tried more gear because, heck, we're new at this (we never planned to qualify for a record on our break-in run), and didn't even consider that the V4F could pull more than we had.  We'll have more gear in October.

I am now very aware that no one recommends R&P steering on the salt.  Too late to modify before October though.  Good winter project.

As for Caster.  I have 9 degrees.  Is more a "crutch"?  How much more?  Consensus (if possible)?

I still must ask.  Is CP/CG an issue at 120 mph?

Keep the great ideas coming!

Thanks, Pedro

[Sumner]

...............I still must ask.  Is CP/CG an issue at 120 mph?..........

It could be an issue at any speed, but my personal feeling is that this mainly comes into play to help the car go straight once it is upset (rear wheels/tires get loose for whatever reason and the back wants to come around) and is why I asked if you thought you were spinning the tires.  Don't use CP/CG as a crutch if at all possible, but then again a short wheelbase roadster needs all the help it can get as it is about as close to a "top" on wheels as you can get and that is mostly because it is hard to get the CG very far ahead of the CP if in fact you can do that. 

You say you don't have any ballast, so I guess that means there are no heavy items behind the rear axle such as a battery??  If so try moving them ahead of the axle.

Get the scrub radius better and I'll bet you do better.  There again it will get worst when you are in a low accelleration mode.

c ya,

Sum

[Stan Back]

Pedro --

Sent you a PM.

Stan

[jl222]

  Pedro

  We have run 288mph with a 279mph exit speed and run 65% weight on rear tires, batteries and weight boxes behind rear tires. car engineered by pros [Alston]
 Car is coupe with spoiler to combat lift -3000lbs on rear tires.
 All cars with out spoilers our wings have lift and i believe roadster are among the worst, i wish more of the really fast roadster guys would post what they were running for weight and wheel %.
           
                              JL222

[Pat Kinne / Salt201]

In a quick look I saw no mention of what type of rear end (Ford) you are running - is it open, spool or "locker" - As J.D. pointed out this is a factor - poorly functioning "lockers" can give you fits darting left and right. Real chassis guys will give you reasons why open or spool is better. Pat

[V4F STR 60]

OK, more answers, more questions...

Stan I did get your PM.  Thank you.  I'll call you today or tomorrow for the wheel info.

Sum, I do not know if the rear tires were spinning or not.  Being a newbie with a loose car, I was a bit more focused on the car going straight than the tach.  I will have tach recall next time out.

Pat, The rear is open.  We never thought spinning the tires might even be an issue in a V4F when we built the car.  Heck, we hadn't even fired the engine 72 hours before we loaded it on the trailer.

There is no added weight behind the rear axle.  Battery and H20 tank are in the cockpit.

The overwhelming consensus at this point seems to be more gear and less scrub.

Still whats the magic caster #?

Are Moon tanks of any aero benefit?  Surprised I don't see more of them on STRs.

Thanks guys!

Pedro

[4-barrel Mike]

Pedro:  Welcome to the V4F world.

The #1931 coupe had 10% slippage (RPM vs speed) on its best run this year.

Tony Thacker wind tunnel tested the Thacker & Shine STR in Hot Rod here: http://www.hotrod.com/roadtests/hrdp_0604_thacker_shine_a_v8_roadster_testing/index.html

The conclusion on their car was that the Moon tank provided less aero.  YMMV

Mike

[jl222]

Pedro
You don't think lift is a factor?

[jl222]

Pedro:  Welcome to the V4F world.

The #1931 coupe had 10% slippage (RPM vs speed) on its best run this year.

Tony Thacker wind tunnel tested the Thacker & Shine STR in Hot Rod here: http://www.hotrod.com/roadtests/hrdp_0604_thacker_shine_a_v8_roadster_testing/index.html

The conclusion on their car was that the Moon tank provided less aero.  YMMV

Mike

 Hi Mike

Thanks for the link,thats the most info i have ever read about roadster aero , very enlightning - roll cage levering weight on rear end - cd-.521-lift on front -on and on-great! Now going to reread.

                             JL222